Conventionally, sugar is obtained from beets by first cleaning the harvested beets, which frees them of a majority of the earth still adhering and of leaf residues. After passing through a washing step, the beets are sliced to pencil-thick chips by cutting machines. The sugar is obtained from the chips by countercurrent extraction using hot, slightly acidified water having a pH of about 5.5 to 5.8. The acidification of the extraction liquid promotes the filtration of the sugar beet raw juice and the ability of the extracted chips to be pressed out. The sugar beet raw juice obtained in the extraction is subsequently sent to an extract purification. Typically, the extract is purified with the aid of so-called lime-carbonic acid extract purification in the form of a preliming step and main liming step, and of a first and second carbonation step and of the removal of the precipitate after the first and second carbonation. The extract purification has the task of very substantially removing the non-sucrose substances present in the sugar beet raw juice, especially high molecular weight substances. The non-sucrose substances to be removed should as far as possible not be degraded, so that no additional low molecular weight substances get into the extract or sugar beet raw juice.
In the preliming step, the sugar beet raw juice is alkalized by adding milk of lime stepwise under gentle conditions. This raises the pH of the sugar beet raw juice in the preliming reactor stepwise to about 11.5. The preliming is effected with addition of defined amounts of calcium hydroxide (milk of lime), the alkalinity of the juice at the end of the preliming step being about 0.1 to 0.3 g of CaO/100 ml of sugar beet raw juice. Owing to the alkalization of the sugar beet raw juice, there is neutralization of the organic and inorganic acids present in the extract, and there are also precipitation reactions of the anions which form insoluble or sparingly soluble salts with calcium. For example, phosphate, oxalate, citrate and sulfate are very substantially separated out. In addition, colloidally dissolved non-sucrose substances coagulate and are precipitated out. The precipitation of individual ingredients, for example of anions such as oxalate, phosphate, citrate, sulfate or of colloids such as pectin and protein substances proceeds within certain pH ranges. Within these pH regions, compaction of the precipitate takes place simultaneously. As a result of the addition of milk of lime during the preliming, there is also coagulation of proteins.
The object of the main liming performed subsequently consists especially in the chemical degradation of invert sugar and amides, which would otherwise proceed in the region of the juice thickening with formation of acid. In the main liming step, the temperature is raised to about 85° C. and the alkalinity of the sugar beet raw juice is increased significantly by the addition of milk of lime, specifically to about 0.8 to 1.1 g of CaO/100 ml of juice. The processes desired in the main liming proceed in the classical process only under such drastic conditions. The lime added in excess in the main liming also plays a large part in the first and second carbonation. Conversion to calcium carbonate provides a strong adsorbent for a series of soluble non-sucrose substances and also a suitable filter assistant. The lime unconsumed in the main liming process is converted to calcium carbonate by introduction of carbon dioxide as a carbonation gas in the two carbonation steps. The carbonation is effected in two stages. In the first stage of the carbonation, gas is introduced up to a pH of about 11.2 to 10.6, which corresponds to an alkalinity of 0.10 to 0.06 g of CaO/100 ml of filtrate of the first carbonation. In the first carbonation, the non-sucrose substances which have precipitated out and flocculated out, and a portion of the dyes present in the sugar beet raw juice are bound adsorptively to the calcium carbonate formed. The so-called first carbonated juice obtained in the first carbonation is filtered through thickening filters (candle filters) or decanters passed, and thickened to carbonated juice concentrate. This removes the non-sucrose substances which have precipitated out and flocculated out and are bound to calcium carbonate from the juice. Typically, the first carbonation is followed by a postliming step, in which the juice is admixed with a little milk of lime and then carbonated further in the second carbonation step. In the second carbonation stage too, carbonation gas is supplied, the alkalinity to be established, known as the optimal alkalinity, being 0.025 to 0.010 g of CaO/100 ml of filtrate of the second carbonation. The alkalinity corresponds to a pH of about 9.0 to 9.30. In the second carbonation, the so-called second carbonated juice forms, which is likewise filtered through thickening filters and thickened. The calcium carbonate sludges (carbonated sludge concentrates) concentrated by means of the thickening filters in the first and second carbonation are typically combined and pressed out by means of membrane filter presses. This forms the so-called carbolime. This carbolime is a storable product having a dry substance content of more than 70% and is used as a fertilizer. Typically, a portion of the carbonated juice concentrate is returned to the preliming.
The sugar beet raw juice purified in the extract purification, which is also referred to as thin juice and contains about 15 to 17% sugar, is then thickened to a thick juice with about 65 to 70% sugar and then concentrated in crystallizers until a viscous mass, known as the massecuite, with about 85% sugar. Centrifugation of this mass then finally affords white sugar.
A considerable disadvantage of the conventional lime-carbonic acid extract purification is in particular that only a relatively small purification effect is achieved, since only a maximum of 40% of all non-sucrose substances are removed from the sugar beet raw juice. A further disadvantage is that the process requires very large amounts of lime, and the amount of lime consumed can make up about 2.5% of the total weight of the processed sugar beets. The production of the lime used in the lime-carbonic acid extract purification process and the disposal of the waste formed in the quicklime production are, however, relatively expensive. The CO2 emissions from limekiln and juice purification plant are also very high. Furthermore, the carbolime which is obtained in the lime-carbon dioxide extract purification process and consists of lime and removed juice impurities can be used only as fertilizer.
The technical problem on which the present invention is based is therefore to provide an improved process for extract purification of sugar beet raw juice, in which especially the lime consumption for the extract purification is reduced.